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There is increasing demand for pipeline installation, including SCRs, in deeper water, coupled with a requirement for higher operating pressures and temperatures and the need to transport corrosive constituents. For such applications, the use of high strength steel, Grade X80, offers significant benefits including a reduction in pipeline weight and savings in material and fabrication costs. Furthermore the reduction in linepipe weight reduces buoyancy module requirements and facilitates installation by existing pipelay vessels which would otherwise require increased top tension capability if lower strength pipe was used.Reel-lay offers a cost effective offshore installation method for high strength steel pipe. Hitherto reel-lay installation has been limited to Grade X65/70 strength pipe. Subsea 7, in collaboration with Vallourec and Mannesman Tubes, (refer to hereafter as V&M Tubes) has performed a qualification programme for reelable X80 linepipe. V&M Tubes manufactured seamless X80 pipe of 323.9mm OD x 18mm WT pipe in accordance with DNV OS-F101, supplementary P requirements. Subsea 7 developed and qualified a mechanised girth weld procedure based on the GMAW-CMT/PGMAW welding process. Procedure qualification was successfully performed in compliance with DNV OS-F101, including mechanical, fracture toughness and sour service testing.In order to address the need to transport more corrosive constituents, Butting manufactured Alloy 625 and 316L mechanically lined or BuBi ® pipe( 323.9 x17.5+3.0mm) using the X80 pipe supplied by V&M Tubes. Subsea 7 developed a novel girth welding procedure utilising internal welding of the CRA lining and external welding using conventional C-Mn filler wire. The latter facilitated the achievement of overmatching weld metal strength which is necessary for reeled pipe. Girth weld procedure qualification was successfully performed in accordance with DNV OS-F101 including a full scale bending trial.The development of linepipe material and welding solutions for reelable high strength carbon steel and CRA lined pipe are considered to be key enabling technologies for the exploitation of deep water oil and gas reserves in the future.
The use of mechanically lined pipe (MLP) using a thin liner, i.e. 2.5mm, can provide a more cost effective linepipe material solution relative to a standard 3.0mm liner. This is especially the case for the more expensive liner materials with higher corrosion resistance, including Alloy 625. Thin liners, i.e. 2.5mm, can be used without compromising pipeline integrity and performance, whilst still fulfilling design requirements defined in most pipeline design standards, including DNVGL-ST-F101. The suitability of 2.5mm liner MLP has previously been demonstrated in service over a range of pipeline bundle projects installed with the controlled depth tow method, but not to date for risers installed by reel-lay. This paper presents the details and test results of the qualification programme to support its use for both flowlines and risers installed by reel-lay. The qualification MLP test pipes, which comprised an outer diameter (OD) 219.1mm × wall thickness (WT) 15.9mm X65 + 2.5mm Alloy 625, were manufactured using established manufacturing procedures and facilities. Reeling and fatigue test strings were fabricated using qualified welding solutions. The fabricated test strings were subject to internal visual inspection and dimensional measurement using laser metrology in order to provide a benchmark for comparison post reeling. The test strings were given a simulated reeling procedure using bending and straightening formers representative of a reel-lay vessel with the smallest reel hub diameter, this being a conservative material straining condition. An internal pressurisation technique, as per standard installation practice for the present pipe lay contractor for MLP, was applied during the simulated reeling procedure. Post reeling the internal laser metrology inspection procedure was repeated in order to confirm the integrity of the liner and to check for the presence of any evidence of liner wrinkling or damage. Subsequently, full scale fatigue testing was performed using the high frequency resonance bending procedure. Testing was performed to ultimate failure to determine the fatigue endurance limit of the thin liner MLP. Additionally Finite Element Analysis (FEA) was performed to further validate the satisfactory reeling performance of the thin liner MLP. The FE numerical analysis embraced manufacture of the MLP pipe and test samples coupled with the reeling procedure. Sensitivity analysis on pipe strength and geometrical mismatch was performed to demonstrate the robustness of the linepipe material solution and reeling procedure. All of the critical qualification activities were performed and verified by DNVGL and in accordance with the guidance of DNVGL-RP-A203 Technology Qualification process. The paper highlights the qualification programme performed to enable the cost effective use of thin liner MLP, specifically Alloy 625, for risers installed by reel-lay.
The use of mechanically lined pipe (MLP) for both flowlines and risers installed by reel-lay is well established, giving significant cost and schedule benefits relative to conventional metallurgically clad pipe. Successful offshore installation of MLP is underpinned by comprehensive qualification testing. Evolving MLP products, including the use of thin liners and adhesively bonded MLP, i.e. GluBi®, continue to improve the competitiveness of this linepipe product. The paper will highlight the key steps for qualification, including new products, the lessons learnt captured during fabrication and installation as well as the benefit of a local spool base for the Asia Pacific region. Subsea 7 has worked in close collaboration with leading supplier Butting to perform the qualification of MLP. The latter, initially, comprises extensive non-destructive and destructive testing of the linepipe materials including the liner/ weld overlay interface. Subsequently reeling test strings are fabricated using qualified welding solutions. Internal visual inspection and dimensional measurements are carried out using laser metrology to provide a benchmark for comparison post reeling. The test strings are given a simulated reeling procedure using bending and straightening formers, representative of Subsea 7's installation vessels. The internal pressurisation technique, as per standard installation practice for MLP, is applied during the simulated reeling procedure. The need for internal pressurisation is eliminated in the case of adhesively bonded MLP. Post reeling the internal laser metrology inspection procedure is repeated to confirm the integrity of the liner and to check for the presence of any evidence of liner wrinkling or damage. Subsequently, for riser applications, full-scale fatigue testing is performed using the high- frequency resonance bending procedure with a focus on the integrity of the junction between liner and weld overlay or, as commonly termed, the triple point. Additionally, finite element analysis (FEA) is often performed to further validate the satisfactory reeling performance of the MLP. All the qualification activities are carried out and verified in alignment with DNV-RP-A203 Technology Qualification (Ref.1) To date Subsea 7 has installed several hundreds of kilometers of MLP flowlines and risers, with pipe NPS in the range 7" to 14" and including 316L, Alloy 825 and Alloy 625 liners. This thorough qualification process and experience combined with the successful set up of a regional spool base provides a robust and cost-effective alternative to the conventional metallurgically clad pipe.
With the increasing development of high temperature/high pressure wells, particularly in deep water, riser designs using conventional strength material are utilising increasingly heavier wall thickness pipe, up to 50mm wall thickness in 8" and 10" OD X65 linepipe. Such riser designs are challenging existing seamless linepipe manufacturing and girth welding capabilities. Consequently, Subsea 7 undertook a linepipe material and welding qualification programme in order to provide confidence in the use of heavy wall pipeline and riser designs and installed by R-lay. Riser designs with heavy wall thickness may impose excessive top tension requirements making them difficult to install cost effectively using R-lay technology. Additionally, such risers may pose excessive payloads on the floating production vessel. Fatigue requirements at the hang off and touch down zones pose further limitations on the use of heavy wall risers. These limitations may impede cost effective development of HP/HT fields. The application of high strength steel for HT/HP applications as riser material is an attractive alternative. High strength steel reduces the wall thickness and thereby reduces payloads on floating hostfacility and top tension requirements for R-lay installation. Application of high strength steel improves the top end interface design also, due to reduced tension. The qualification status of reelable X80, including CRA lined pipe will be presented. Another challenge for HP/HT riser application is the top end connection to the floater. Traditional flex-joint solutions may not be feasible, requiring a Stress Joint at the top. For ultra-deep water conditions, special stress joints with titanium may be necessary. The limitation and application of such top end solutions for HP/HT application will be presented in this paper.
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